Beat the eggs and sugar on medium-high speed for 15 minutes. Add the cake meal and potato starch, beating at the lowest speed until mixed in. Melt the chocolate (you can melt it while beating the eggs and sugar). Pour 3/4 of the batter into an ungreased 10-inch bundt or tube cake pan. Add the melted chocolate to the remaining 1/4 of the batter and stir until evenly mixed. Spoon the chocolate batter into the pan and swirl it through with a knife.

Bake for 50 to 55 minutes. Let cool for 10 minutes on wire rack, then invert cake onto rack to cool completely. Dust with confectioners' sugar before serving, if desired.